1. Field of the Invention
The invention is directed to a method for the adaptive optimization of ultrasound measurement signals.
2. Description of the Prior Art
The measuring precision and the structural resolution of ultrasound pulse-echo systems is critically determined by the spectral system bandwidth and by the chronological shape of the measurement signal. For example, pulse-shaped measurement signals with a broad frequency response and optimally short build-up and decay times are desirable for a high measuring precision and good axial resolution. In standard ultrasound transducers, it is usually a matter of weakly attenuated, resonant transducers that preferably oscillate at specific modes and, due to their mismatch to the acoustic medium of air, exhibit a limited efficiency/bandwidth product. For this reason, a pulse-shaped excitation of these transducers does not as a rule lead to measurement signals that are optimum in the above sense.
The, reprint from Fachberichte Huttenpraxis Metall-Weiterverarbeitung. volume 19, No. 2, 1981, H. A. Crostak, "Grundlagen des CS-Technik", discloses a possibility for improving the transmission signal of an ultrasound transducer for materials' testing with ultrasound. This ensues by the variation of the frequency spectrum of the transmission signal, for example by multiplication of a pulse by one or more selected functions.
It is fundamentally possible to partially compensate the non-ideal transmission properties of the transducers with a suitable filtering. What are referred to as inverse filters can be applied in the field of ultrasound technology in order to improve the characteristic data of ultrasound measurement systems.
Such an inverse filter is disclosed in, Sedki M. Riad, "The Deconvolution Problem: An Overview", IEEE, 1986. This is thereby a matter of a post-filtering.
In inverse filtering, the ultrasound signal output by a transmission transducer is received and subsequently charged with the inverse transfer function of the system.
A further possibility is to calculate an inverse filter function into the signal to be transmitted. This represents an inverse pre-filtering.
Given post-filtering, the reception signals must be filtered after every measurement. Since analog variable filter can be realized only with great difficulty, this filtering usually ensues with digital filters, whereby implementing the filtering in real time is partly possible only with relatively great outlay.
One disadvantage of inverse pre-filtering is the need for a linear, broadband power transmission amplifier for the excitation of the ultrasound transducer. Due to the great outlay and the relatively high power consumption, the use thereof is often unacceptable, particularly given reasonably priced "stand alone" apparatus.
Both inverse pre-filtering as well as post-filtering have the disadvantage that information about the overall system is necessary for the calculation of the inverse filter, these having to be present in advance, and the method very easily becoming unstable when these information are not considered or given imprecise data and/or optimum results cannot be achieved.